In a hot water heating system using a heat pump apparatus, the heat pump apparatus cools circulating water during defrost operation. Thus, various methods for preventing freezing of the circulating water have been invented.
For example, there are methods for preventing the freezing of the circulating water such that an auxiliary heater is always activated during the defrost operation of the heat pump apparatus, or such that the auxiliary heater is activated when a temperature of the circulating water is at or below a freezing determination temperature, and so on.
For a water heater and hot water heating system capable of supplying hot water, there is a method for preventing the freezing of the circulating water by using high-temperature water in a hot water storage tank.
(Use of an Auxiliary Heater)
However, an auxiliary heater requires a large amount of power. For this reason, in terms of saving energy, it is desirable to minimize use of the above-described methods, that is, the method in which the auxiliary heater is always activated during the defrost operation of the heat pump apparatus, or the method in which the auxiliary heater is activated when the temperature of the circulating water is at or below the freezing determination temperature.
(Presence of a Hot Water Storage Tank)
In the above-described method using the high-temperature water in the hot water storage tank, it is a prerequisite that there exists a hot water storage tank in a water heater and hot water heating system capable of supplying hot water. For this reason, this method cannot be applied to a hot water heating system configured with a simple water circuit in which a heat pump apparatus, a circulation pump, and a hot water heating appliance are connected circularly with pipes.
Patent Document 1 relates to prior art for reducing use of the auxiliary heater. Patent Document 1 discloses a control method for preventing an auxiliary heat source from unnecessarily being activated during defrost operation of a heat pump heat source in a hot water heating system. In Patent Document 1, whether to turn on or off the auxiliary heater is determined based on each instance of temperature information of the hot water heating system.
However, the technology discussed in Patent Document 1 controls only the auxiliary heater, so that it is inadequate in terms of reducing use of the auxiliary heater and promoting energy saving.